1. Field of the Invention
This invention relates to a transparent electrode for an electrochromic switchable cell, and more particularly, to a transparent electrode having a coating stack having a pair of electrically conducting layers separated by either an electrically conducting layer or a high electrically resistance layer such as a dielectric layer.
2a. Discussion of the Technology
Electrochromic switchable transparencies are often used when it is desired to vary visible light transmission through a transparency or glazing. For example and not limiting to the discussion, switchable transparencies are used for aircraft windshields and/or side windows to provide the operator of, and/or passengers in, the aircraft with the ability to increase or decrease the visible light transmittance of the transparency.
One type of an electrochromic transparency or system includes anodic compounds and cathodic compounds together in a solution between a pair of spaced electrode assemblies. Another type of system includes a polymer having a cathodic-coloring polymer, an electrically insulating but ionically conductive polymer and an anodic-coloring polymer between a pair of spaced electrode assemblies. Still another system includes an inorganic layer which is ionically conductive but electronically insulating between a pair of spaced electrode assemblies. For a more detailed discussion of the above-mentioned electrochromic systems, reference can be made to U.S. Pat. Nos. 5,202,787; 5,805,330; 6,643,050, and 6,747,779. As is appreciated by those skilled in the art, for an electrochromic mirror, at least one of the electrode assemblies is transparent, and for an electrochromic transparency preferably both electrode assemblies are transparent.
The electrode assembly in one arrangement includes an electrode mounted on the surface of a glass sheet. A pair of the electrode assemblies is mounted in spaced relationship to one another with the electrodes in facing relationship with one another and in electrical contact with the electrochromic medium between the electrodes. The electrodes are connected to electronic circuitry, such that when the electrodes are electrically energized, an electrical potential is applied to the electrochromic medium and causes the medium to change color. For example, but not limiting to the discussion, when the electrochromic medium is energized, it darkens and begins to absorb light to decrease the percent of visible light transmitted through the electrochromic medium, and when the medium is de-energized, it lightens to increase the percent of visible light transmitted through the medium. The percent reduction of visible light through the medium depends on the amount of voltage applied to the medium, e.g. as the applied voltage increases, the percent of visible light transmitted through the medium decreases. For a more detailed discussion on the operation of electrochromic switchable transparencies, reference can be made to above-mentioned U.S. Pat. Nos. 5,202,787; 5,805,330; 6,471,360, and 6,643,050.
The switching speed of the electrochromic switchable medium from maximum to minimum visible light transmission within the electrochromic medium transparency's range of visible light transmission is a function of several parameters discussed in the above-mentioned patents. For example and not limiting to the discussion, one of the parameters is the electrical conductivity of the electrodes, e.g. the sheet resistance of the electrodes contacting the electrochromic switchable medium. The sheet resistance is the resistance in ohms per square of the electrode measured between two points on the surface of the electrode designated to contact the electrochromic medium. The procedure for determining sheet resistance of electrodes is well known in the art, and no further discussion is deemed necessary.
As is appreciated by those skilled in the art, for a given applied voltage, as the sheet resistance of the electrode decreases the switching time from a maximum visible light transmission value to a minimum visible light transmission value decreases and vice versa. The decrease in switching time is a result of increasing the current through the electrodes and the electrochromic switching medium. In general, to obtain a uniform distribution of current between the electrodes, it is desirable for the electrodes to have a low sheet resistance, e.g. below 10 ohms per square.
It is also recognized in the art that temperature variations have an affect on the switching time, e.g. as the temperature decreases, the switching time increases and visa versa.
2b. Discussion of the Presently Available Technology
U.S. Pat. No. 5,805,330 discloses materials that can be used for electrodes of photovoltaic-powered electro-optic devices. The materials include fluorine doped tin oxide, tin doped indium oxide (hereinafter also referred to as “ITO”), thin metal layers, ITO/metal/ITO (hereinafter the combination also referred to as “IMI”), and additional layers of metal and ITO, e.g., IMIMI. U.S. Pat. No. 6,747,779 discloses electrodes of ITO/silver/ITO, silver/ITO and dielectric/silver/ITO with optional interposition of thin layers of partially oxidized metal at the silver-ITO interface.
U.S. Pat. No. 6,561,460 discloses a fog-preventing electrochromic pane assembly including a first substrate having a conductive coating on an outer first surface, and second conductive coating on an inner second surface, as well as a second substrate spaced from the first substrate, with the second substrate having a third conductive coating on outer first surface. The inner second surface of the first substrate and the outer first surface of the second substrate face each other in spaced-apart relation to define a chamber therebetween having an electrochromic medium. Electrical current is applied to the first conductive coating to heat the first conductive coating, which prevents fogging of the pane assembly, and an electrical potential is applied to the second and third conductive coatings to vary the transmittance of the electrochromic medium.
Although the fog-preventing electrochromic pane assembly of U.S. Pat. No. 6,561,460 is acceptable for its intended purpose, there are limitations. More particularly, a sheet having a coating on both surfaces has to be handled carefully to prevent surface damage to one or both of the coated surfaces.
As can be appreciated, it would be advantages to provide electrodes that provide a uniform distribution of current through the electrochromic medium, that provide facilities to heat one or more of the outer surfaces of the electrochromic switchable transparency, and/or that enhance solar protection of the electrochromic medium without having the limitations of the presently available transparent electrodes for electrochromic transparencies.